<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>常用控制语句—if、if else</title>
</head>
<body>
<script>
  let  num = 15;
  if(num==20) {
    //
    console.log("if内代码块");
  }else{
    //
    console.log("else内代码块");
  }
</script>
</body>
</html>